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I need to use a reed switch and a magnet on a shaft. When it get turned to a specific degree the magnet will trigger the reed switch. The shaft only turns one way and is turned by hand. The problem is the device could have a lot of vibrations; like falling to the ground or getting kicked and punched—kids involved :) Now if a really bad knock occurs I can live with some false positives but those will be deferred as best as possible.

How can I find the correct reed switch and magnet combination. How do I match these parameters up? I know I need to select the correct reed switch but what do I need to look for in terms of identifying the "stiffness"/"tension" of the plates? Then I can just buy magnets and test them by trial and error?

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    \$\begingroup\$ Is a reed switch a hard requirement? Would you consider a hall-effect sensor or even an optical sensor? \$\endgroup\$ – Dave Tweed Feb 14 '13 at 12:10
  • \$\begingroup\$ The only requirement it needs to be as cheap as possible with minimal components on a battery operated system. But I will look into hall-effect - optical will use too much power :( And it must act like a switch. button press equivalent \$\endgroup\$ – Piotr Kula Feb 14 '13 at 12:12
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    \$\begingroup\$ Interesting question, I've never looked at reed switch datasheets before but after a quick look I'd assume you want a high 'AT range' because presumably the ones with the lowest sensitivity are stiffest. But I'm not familiar with that scale so don't know at what point it would get too large for a small magnet. \$\endgroup\$ – PeterJ Feb 14 '13 at 12:16
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    \$\begingroup\$ When searching add 'switch' to the equation. For example at Digikey the following digikey.com/product-detail/en/AH180-PL-A/AH180-PL-ADICT-ND/… is < $1 and gives a digital output if all the rest looks OK. \$\endgroup\$ – PeterJ Feb 14 '13 at 12:50
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    \$\begingroup\$ @Brian Shopping questions are off-topic! :) (And now we know why: top-secret government conspiracy power cells...) \$\endgroup\$ – JYelton Feb 26 '14 at 19:29
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One of my school projects required magnetic sensing, and using a solid state hall switch beat out a mechanical reed switch in terms of price, size and reliability. I recklessly ordered high-sensitivity switches and used a really strong neodymium magnet which did work (perhaps a little too well, turns out the pull up resistors were magnetic too!) but obviously this is a risky approach.

If you have an unknown magnet, you can use a gaussmeter to measure the magnetic flux density at the approximate distance you want to trip and buy a sensor that has an operating point slightly less than that in the worst case. If you're buying magnets, then you want to use a magnet calculator like this one to calculate how strong your magnetic field will be, given the dimensions you've specified for the magnet and the point to measure at, and then buy a sensor in line with that. Magnetic flux densities and operating points are specified in either gauss or millitesla.

NB, the Melexis US5781, a variant of the one I used, has an operating point of 15mT (150 gauss). That's low as far as magnets go. It pulls down as long as the magnet is in proximity.

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    \$\begingroup\$ Fantastic. Thank you very much for a detailed answer. Because my project is power sensitive (i need it to run as long as possible on a chosen battery pack) I opted out of these and instead found some cheap switches. Like two pieces of reinforced copper plate that can withstand a few g's of shock, like shaking the device or whatever. Its cheap, easy and uses 0 power :) Really appreciate you educated insight. I will keep it in mind for next projects and it does answer this question :) +1 \$\endgroup\$ – Piotr Kula Feb 26 '14 at 20:56

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